1. Field of the Invention
The present invention relates generally to a mobile communication system, and more particularly to a method for radio resource management for a User Equipment (UE) operating in both a Radio Resource Control (RRC) connected mode and a Discontinuous Reception (DRX) mode in a mobile communication system, and a UE for the same.
2. Description of the Related Art
A Universal Mobile Telecommunication Service (UMTS) system is a 3rd generation (3G) asynchronous mobile communication system, which uses wideband Code Division Multiple Access (CDMA) and is based on Global System for Mobile Communications (GSM) and General Packet Radio Services (GPRS), which are European mobile communication systems. In the third generation Partnership Project (3GPP), which is in charge of standardization of the UMTS, there is active discussion about a Long Term Evolution (LTE) system as a next generation mobile communication system. The LTE technology is targeting commercialization by 2010 and the realization of high speed packet-based communication at speeds of about 100 Mbps. Accordingly, various schemes are being discussed, which include a scheme of reducing the number of nodes located in communication paths by simplifying the structure of a network, and a scheme of approaching a wireless protocol to a wireless channel as closely as possible.
FIG. 1 illustrates an example of a structure of an evolved UMTS mobile communication system to which the present invention is applicable. Referring to FIG. 1, each of Evolved UMTS Radio Access Networks (E-RANs) 110 and 112 has a simplified 2 node structure, which includes Evolved Node Bs (ENBs) 120, 122, 124, 126, and 128 and anchor nodes (Enhanced Gateway General Packet Radio Service (GPRS) Support Nodes (EGGSNs)) 130 and 132. A User Equipment (UE) 101 is connected to an Internet Protocol (IP) network 114 through the E-RAN 110 or 112. The ENBs 120 to 128 correspond to legacy Node Bs of the UMTS system and are connected to the UE 101 through a wireless channel. However, different from legacy Node Bs, the ENBs 120 to 128 perform more complicated functions. In the LTE, all user traffic including the real-time service, such as Voice over IP (VoIP) using the Internet protocol, are provided through a shared channel. Therefore, the LTE requires an apparatus for collecting status information of UEs and performing scheduling using the collected information. The ENBs 120 to 128 control the scheduling. Usually, one ENB controls a plurality of cells. Further, the ENB performs Adaptive Modulation and Coding (AMC), which determines a modulation scheme and a channel coding rate in accordance with the channel status of a UE. As in the High Speed Uplink Packet Access (HSUPA), which is also called “Enhanced Dedicated Channel (E-DHC)”, and the High Speed Downlink Packet Access (HSDPA) of the UMTS, the Hybrid Automatic Repeat Request (HARQ) is performed between the ENBs 120 to 128 and the UE 101 in the LTE also. The HARQ process soft-combines previously-received data with retransmitted data without discarding the previously-received data, thereby improving the ratio of success in the reception. The HARQ process improves the transmission efficiency in the high speed packet communication, such as the High Speed Downlink Packet Access (HSDPA) and the Enhanced Dedicated Channel (EDCH). In order to implement a maximum transmission speed of 100 Mbps, the LTE is expected to use Orthogonal Frequency Division Multiplexing (OFDM) in 20 MHz bandwidth as wireless connection technology. However, because it is impossible for only the HARQ to satisfy requirements for various Qualities of Service (QoSs), an outer Automatic Repeat Request (ARQ) in a higher layer may be performed between the UE 101 and the ENBs 120 to 128.
In the wireless communication as described above, degradation in the quality of a high quality data service is caused mainly by the channel environment. The channel environment in the wireless communication frequently changes due to interference by multi-path signals or other users, Doppler Effect due to movement and frequent speed change of a User Equipment (UE), shadowing, change in the power of a received signal caused by the fading as well as the Additive White Gaussian Noise (AWGN), etc. One of the main schemes used to overcome the fading in a typical OFDM system is the Adaptive Modulation and Coding (AMC) scheme. According to the AMC scheme, the modulation scheme and the coding scheme are adaptively controlled according to channel change in a downlink (DL).
In order to apply power control or an AMC scheme according to the channel change to a DL channel, a UE must report Channel Quality Information (CQI) of a received downlink signal to the ENB. Usually, it is possible to detect the CQI by measuring a Signal to Noise Ratio (SNR) of a received signal by a UE. Upon receiving the CQI from the UE, the ENB can acquire the information on the downlink channel state of the UE and can set a corresponding modulation scheme and a corresponding coding scheme or control the power based on the acquired DL channel state information. In contrast, in order to apply the power control or AMC scheme according to the channel change to the UL channel, the UE must transmit a pilot signal (also called a “Reference Signal (RS)”) with a predetermined pattern in an uplink. Hereinafter, such a UL pilot signal is called a “sounding.” The ENB can measure the uplink channel state through a received sounding and can set a corresponding modulation scheme and a corresponding coding scheme or control the power based on the measured uplink channel state.
The CQI, sounding, or a response information, such as an Acknowledgement/Negative Acknowledgement (ACK/NACK), on an uplink HARQ in response to transmission of a downlink HARQ is commonly referred to as “uplink control information”.
FIG. 2 illustrates an example of a Discontinuous Reception (DRX) operation of a UE that is in a Radio Resource Control (RRC)-connected mode.
In the 3rd Generation Partnership Project (3GPP), radio modes of a UE in the 3GPP LTE system are classified largely into an RRC idle mode and an RRC connected mode. Definitions of the RRC idle mode and the RRC connected mode are based on 3GPP TS36.300.
In general, the RRC idle mode refers to a state of a UE, in which the ENB does not have information of Radio Bearer (RB) context and UE context, and an anchor node has the context information of the UE and manages the location of the UE according to each Tracking Area (TA) for paging, instead of managing the location cell by cell. Further, the RRC connected mode refers to a state of a UE, in which not only an anchor node, but also the ENB has information of RB context and UE context (a possibility that the information may include service context information is not excluded). An RRC connection is established between the UE and the ENB, so that it is possible to manage the location of the UE cell by cell. Usually, in order to receive and/or transmit data for a particular service, UEs in an RRC idle mode must first establish an RRC connection to the ENB and report UE context information to the ENB, and then establish a signaling connection to an anchor node and report UE context and service context information to the anchor node. However, UEs in an RRC connected mode can be allocated corresponding radio resources directly from the ENB and then receive and/or transmit data for a particular service through the resources.
The DRX operation minimizes power consumption of a UE through discontinuous reception of channels only at particular periods, instead of continuously consuming power through continuous reception of channels, when it is unnecessary to continuously transmit data to the UE. The DRX usually includes the following elements.                Active Period: a period in which a receiver of a UE is on, or a period in which reception of data of a corresponding service is expected when discontinuous reception has been set for each service.        Sleep Period: a period in which a receiver of a UE is off, or a period in which reception of data of a corresponding service is not expected when discontinuous reception has been set for each service. If the UE receiver is off is determined by the sleep period overlapping an active period of another service.        Discontinuous Reception Period (DRX cycle length; 210 & 220): a period or length between active periods.        
Although FIG. 2 illustrates active periods having the same length, the active periods starting from the points 205, 215, and 225 may have different lengths.
When a UE is in a discontinuous reception mode as described above, uplink control information may not be required as much as in a continuous reception mode. For example, if a radio resource allocated in a continuous reception mode in order to transmit CQI frequently overlaps with a sleep period in a discontinuous reception mode, it is not desirable in view of the power consumption of a UE to transmit multiple CQIs or transmit CQI multiple times in the sleep period. Further, because downlink transmission does not occur in the sleep period, the multiple CQIs are not actually used. Further, even though the UE does not actually perform the transmission of the multiple CQIs, the radio resources for transmission of the CQI or CQIs have already been allocated, which results in a waste of radio resources.
Therefore, it is necessary to reconfigure uplink control information transmission resources, which have been allocated in a previous continuous or discontinuous reception mode, when a UE enters a new discontinuous reception mode. Accordingly, it is necessary to perform an explicit reconfiguration procedure through an RRC message, which may cause a signaling overhead.